In the past 20 years, polylatic acid (PLA) has become a leading biodegradable/compostable polymer for preparation of plastics and fibers. This is because although the PLA is derived from natural and renewable materials, it is also thermoplastic and can be melt extruded to produce plastic items, fibers and fabrics with good mechanical strength and pliability comparable to oil-based synthetics such as polyolefins (polyethylene and polypropylene) and polyesters (polyethylene terephthalate and polybutylene terephthalate). PLA is made from lactic acid, which is a fermentation byproduct obtained from corn (e.g. Zea mays), wheat (e.g. Triticum spp.), rice (e.g. Oryza sativa), or sugar beets (e.g. Beta vulgaris). When polymerized, the lactic acid forms a dimer repeat unit with the following structures:

Unlike other synthetic fiber materials (such as cellulosics) originated from plant, PLA is more suited for melt spinning into fibers. Compared to the solvent-spinning process required for synthetic cellulosic fibers, PLA fiber made by adoption of melt spinning allows for lower economic cost and environmental cost, and the resulting PLA has a wider range of properties. Like polyethylene terephthalate polyester (PET), PLA polymer needs to be dried before melting to avoid hydrolysis during melt extrusion, and fiber from both polymers can be drawn (stretched) to develop better tensile strength. The PLA molecule tends to form a helical structure which brings about easier crystallization. Also the lactic dimer has three kinds of isomers: an L form which rotates polarized light in a clockwise direction, a D form which rotates polarized light in a counter-clockwise direction and a racemic form which is optically inactive. During polymerization, the relative proportions of these forms can be controlled, resulting in relatively broad control over important polymer properties. The control over a thermoplastic “natural” fiber polymer, unique polymer morphologies and the isomer content in the polymer enables the manufacturer to design a relatively broad range of properties in the fiber (Dugan, J. S. 2001, “Novel Properties of PLA Fibers”, International Nonwovens Journal, 10 (3): 29-33; Khan, A. Y. A., L. C. Wadsworth, and C. M. Ryan, 1995, “Polymer-Laid Nonwovens from Poly(lactide) Resin”, International Nonwovens Journal, 7: 69-73).
PLA is not considered to be directly biodegradable in its extruded state. Instead, it must first be hydrolyzed before it becomes biodegradable. In order to achieve hydrolysis of PLA at significant levels, both a relative humidity at or above 98% and a temperature at or above 60° C. are required simultaneously. Once these conditions are met, degradation occurs rapidly (Dugan, J. S. 2001, “Novel Properties of PLA Fibers”, International Nonwovens Journal, 10 (3): 29-33 and Lunt, J. 2000, “Polylactic Acid Polymers for Fibers and Nonwovens”, International Fiber Journal, 15: 48-52). However, the melt temperature can be controlled between about 120° C. and 175° C. so as to control the content and arrangement of the three isomers, in which case the polymer is completely amorphous under the low melting temperature. Some more amorphous polymers can be obtained after the addition of enzymes and microbes in the melt.
PLA has been used to make a number of different products, and factors that control its stability and degradation rate have been well documented. Both the L-lactic acid and D-lactic acid produced during fermentation can be used to produce PLA (Hartmann, M. H., 1998, “High Molecular Weight Polylactic Acid Polymers”, p. 367-411, In: D. L. Kaplan (ed.), Biopolymers from Renewable Resources, Springer-Verlag, New York). One advantage of PLA is that the degradation rate can be controlled by altering factors such as the proportion of the L and D forms, the molecular weight or the degree of crystallization (Drumright, R. E., P. R., Gruber, and D. E. Henton, 2000, “Polylactic Acid Technology” Advanced Materials. 12: 1841-1846). For instance, Hartmann (1998) finds that unstructured PLA sample will rapidly degrade to lactic acid within weeks, whereas a highly crystalline material can take months to years to fully degrade. This flexibility and control make PLA a highly advantageous starting material in the production of agricultural mulch fabrics, where the PLA material is intended to be degraded in the field after a specific time period (Drumright, R. E., P. R., Gruber, and D. E. Henton, 2000, “Polylactic Acid Technology” Advanced Materials. 12: 1841-1846).
PLA is decomposed into smaller molecules through a number of different mechanisms, and the final decomposition products are CO2 and H2O. The degradation process is influenced by temperature, moisture, pH value, enzyme and microbial activity while keeping free of being affected by ultraviolet light (Drumright, R. E., P. R., Gruber, and D. E. Henton, 2000, “Polylactic Acid Technology” Advanced Materials. 12: 1841-1846; Lunt, 2000). In some early work that evaluated PLA degradation for biomedical applications, Williams (1981) finds that bromelain, pronase and proteinase K can accelerate the decomposition rate of PLA (Williams, D. F., 1981, “Enzymic Hydrolysis of Polylactic Acid,” Engineering in Medicine. 10: 5-7). More recently, Hakkarainen et al. (2000) incubate PLA sample of 1.8 millimeter thickness at 86° F. in a mixed culture of microorganisms extracted from compost (Hakkarainen, M., S. Karisson, and A. C. Albertsson, 2000., “Rapid (Bio)degradation of Polylactide by Mixed Culture of Compost Microorganisms—Low Molecular Weight Products and Matrix Changes”, Polymer. 41: 2331-2338). After 5 weeks of incubation, the compost-treated film has degraded to a fine powder, whereas the untreated control remains intact. It is noted that this study uses only the L form while the degradation rate will differ based on the ratio of the D and L forms. Regardless, the work by Hakkarainen et al. (2000) illustrates that application of large quantities of readily available microorganisms from compost can accelerate the decomposition. Yet the PLA degradation studies so far are either performed in liquid culture in vitro or in active composting operations above 140° F. (Drumright et al., 2000; Hakkarainen et al., 2000; Lunt, 2000; Williams, 1981). Rapid degradation occurs when PLA is composted at 140° F. with nearly 100% biodegradation achieved in 40 days (Drumright et al., 2000). However, the stability below 140° F. when the fabric is in contact with soil organic matter remains to be determined Spunbond (SB) and meltblown (MB) nonwovens using PLA are first researched by Larry Wadsworth (Khan et al., 1995) at the University of Tennessee, USA (Smith, B. R., L. C. Wadsworth (Speaker), M. G. Kamath, A. Wszelaki, and C. E. Sams, “Development of Next Generation Biodegradable Mulch Nonwovens to Replace Polyethylene Plastic,” International Conference on Sustainable Textiles (ICST 08), Wuxi, China, Oct. 21-24, 2008[CD ROM]).
It is desirable for biodegradable polymers to resist many environmental factors during application period, but to be biodegradable under disposal conditions. The biodegradation of PLA is studied in both aerobic and anaerobic, aquatic and solid state conditions at different elevated temperatures. It is found that in aerobic aquatic exposure, PLA biodegrades very slowly at room temperature but faster under thermophilic conditions. This also supports the findings above that PLA must be hydrolyzed before microorganism can utilize it as a nutrient source. The biodegradation of PLA is much faster in anaerobic solid state conditions than that in aerobic conditions at the same elevated temperatures. In a natural composting process, the behavior of PLA is similar to the aquatic biodegradation exposure, in which biodegradation only starts after it is heated up. These results reinforced a widely held view that PLA is compostable and is stable under room temperature, but degrades rapidly during disposal of waste in compost or anaerobic treatment facilities (Itavaara, Merja, Sari Karjomaa and Johan-Fredrik Selin, “Biodegradation of Polylactide in Aerobic and Anerobic Thermophilic Conditions,” Elsevier Science Ltd., 2002). In another study, the biodegradation levels of different plastics by anerobic digested sludge are determined and compared with those in simulated landfill conditions. Bacterial poly 93-hydroxyvalerate (PHB/PHV), natural aliphatic polyester produced by bacteria, almost completely degrades in 20 days in anaerobic digested sludge; whereas, PLA, the aliphatic polyester synthesized from natural materials, and two other aliphatic polyesters evaluated, poly (butylenes succinate) and poly (butylenes succinate-co-ethylene succinate) fail to degrade after 100 days. A cellulosic control material (cellophane) degrades in a similar way to that of PHB/HV within 20 days. Furthermore, PHB/HV degrades well within 6 months in simulated landfill conditions (Shin, Pyong Kyun, Myung Hee Kim and Jong Min Kim, “Biodegradability of Degradable Plastics Exposed to Anaerobic Digested Sludge and Simulated Landfill Conditions,” Journal of Polymers and the Environment, 1566-2543, Volume 5, Number 1, 1997).
In the search for truly biodegradable polymer, polyhydroxyalkonates (PHAs) have been found to be naturally synthesized by a variety of bacteria as an intracellular storage material of carbon and energy. As early as the 1920s, poly[(R)-3-hydroxybutyrate] (P(3HB)) is isolated from Bacillus megaterium and identified later as a microbial reserve polyester. However, P(3HB) does not have important commercial value since it is found to be brittle and stiff over a long period and thus cannot be substituted for the mainstream synthetic polymers like polyethylene (PE) and polystyrene (PS). Eventually, the discovery of other hydroxalkonate (HA) units other than 3HB in microbial polyesters which can improve the mechanical and thermal properties when incorporated into P(3HB) have a major impact on research and commercial interests of bacterial polyesters. Their biodegradability in natural environment is one of the unique properties of PHA material. The microbial polyester is biodegradable in soil, sludge or sea water. Since PHA is a solid polymer with high molecular weight, it cannot be transported through the cell wall as a nutrient. Thus, the microorganisms such as fungi and bacteria excrete an enzyme knows as PHA degrading enzyme for performing extracellular degradation on PHA. Such enzyme hydrolyzes the solid PHA into water soluble oligomers and monomers, which can then be transported into the cell and subsequently metabolized as carbon and energy sources (Numata, Keiji, Hideki Abe and Tadahisa Iwata, “Biodegradability of Poly(hydroxalkonate)Materials,” Materials,2, 1104-1126, 2009). A random copolyester of [R]-3-hydroxybutyrate and [R]-3-hydroxyvalerate, P(3HB-co-3HV), is commercially produced by Imperial Chemical Industries (ICI) in the UK. It is shown that Alcaligenes eutrophus produces an optically active copolyester of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) by using propionic acid and glucose as the carbon sources (Holmes, PA, (1985), “Applications of PHB: a Microbially Produced Biodegradable Thermoplastic,” Phys Technol 16:32-36 from Kunioka, Masao, Yasushi Kawaguchi and Yoshiharu Doi, “Production of Biodegradable copolyesters of 3-hydroxybutyrate and 4-hydroxybutyrate by Alcaligenes eutropus,” Appl. Microbiol Biotechnol (1989) 30: 569-573). The chemical structure for P(2HB-co-3HV) is as follows:

Furthermore, 3-hydroxypropionate, 4-hydroxyvutyrate, and 4-hydroxyvalerate are found to be new constituents of bacterial polyhdroxyalkonates (PHAs) and have gained much attention in a wide range of marine, agricultural and medical applications. More recently, the microbial synthesis of copolyesters of [R]-3-hydroxybutyrate and 4-hydroxybutyrate, P(3HB-co-4HB), by Alcaligenes eutropus, Comamonas and Alcaligens latus have been studied. The chemical structure of P(3HB-co-4HB) is as follows:

When 4-hydroxybutyric acid is used as the only carbon source for Alcaligenes eutrophus, P(3HB-co-34% 4HB) is produced with the content of 34% 4HB, while 4-hydroxybutyric acid in the presence of some additives is used as the carbon source for Alcaligenes eutrophus, P(3HB-co-4HB) copolyester with a large portion of 4HB (60-100 mol%) is produced. It has also been found that Alcaligenes eutrophus produces a random copolymer of P(3HB-co-4HB) with high efficiency in a one stage fermentation process by the usage of sucrose and 1,4-butyrolactone as the carbon source in a nitrogen free environment. The tensile strength of P(3HB-co-4HB) film decreases from 43 MPa to 26 MPa while its elongation increases from 4-444% with the increasing content of 4HB fraction. On the other hand, as the content of 4HB fraction increases from 64% to 100%, the tensile strength of the film increases from 17MPa to 104 Mpa with the increase of 4HB (Saito, Yuji, Shigeo Nakamura, Masaya Hiramitsu and Yoshiharu Doi, “Microbial Synthesis and Properties of Poly(3-hydroxybutyrate-co-4-hydroxybutyrate),” Polymer International 39 (1996), 169-174). Some studies show that the degree of crystallinity of P(3HB-co-4HB) decreases from 55% to 14% as the content of 4HB fraction increases from 0 to 49 mol %, indicating that 4HB unit cannot crystallize in the sequence of 3HB unit and acts as the defect in the P(3HB) crystal lattice. This is probably largely responsible for the reduced brittleness and improved toughness of P(3HB-co-4HB) compared to P(3HB). Also the melting temperature is found to decrease from 178° C. to 150° C. as the content of 4HB fraction increases from 0 to 18 mol % (Kunioka, Masao, Akira Tamaki and Yoshiharu Doi, Crystalline and Thermal Properties of Bacterial copolyesters: Poly(3-hydroxybutyrate-co-3-hydroxvalerate) and Poly(3-hydroxybutyrate-co-4-hydroxybutyrate),” Macromolecules 1988, 22, 694-697). It has also been shown that the biodegradation rate is increased by the presence of 4HB unit in P(3HB-co-4HB) (Kunioka, Masao, Yasushi Kawaguchi and Yoshiharu Doi, “Production of Biodegradable copolyesters of 3-hydroxybutyrate and 4-hydroxybutyrate by Alcaligenes eutropus,” AppL Microbiol Biotechnol (1989) 30: 569-573). In another study, the enzymatic degradation of P(3HB-co-4HB) film is performed at 37° C. in a 0.1 M phosphate buffer of extracellular depolymerase purified from Alcaligenes faecalis. It is then found that the rate of enzymatic degradation notably increases with the increasing content of 4HB fraction and the highest rate occurs at 4HB of 28 mol % (Nakamura, Shigeo and Yoshiharu Doi, “Microbial Synthesis and Characterization of Poly(3-hydroxybutyrate-co-4hydroxybutyrate),” Macromolecules, 85 (17), 4237-4241, 1992).
This may be due to the resultant decrease in crystallinity; whereas, the presence of 4HB in excess of 85 mol % in the copolyester suppresses the enzymatic degradation (Kumaai, Y Kanesawa, and Y. Doi, Makromol. Chem., 1992, 193, 53 through Nakamura, Shigeo and Yoshiharu Doi, “Microbial Synthesis and Characterization of Poly(3-hydroxybutyrate-co-4hydroxybutyrate),” Macromolecules, 85 (17), 4237-4241, 1992). In a comparison of the biodegradation rates of P(3HB-co-9% 4HB), P(3HB) and P(HB-co-50% 3HV) films, the P(3HB-co-9% 4HB) is found to be completely degraded in activated sludge in two weeks with the degradation rate of this biopolyester being much faster than those of the other two. The degradation rate of P(3HB) is much faster than that of P(HB-co-50% 3HV) film (Kunioka, Masao, Yasushi Kawaguchi and Yoshiharu Doi, “Production of Biodegradable copolyesters of 3-hydroxybutyrate and 4-hydroxybutyrate by Alcaligenes eutropus,” Appl. Microbiol Biotechnol (1989) 30: 569-573).
Polybutylene adipate terephthalate (PBAT) is a biodegradable polymer which is currently synthesized from oil-based products rather than from bacteria. Although PBAT has a melting point of 120° C. which is lower than that of PLA, it has high flexibility, excellent impact strength and good melt processibility. Furthermore, several studies about the biodegradation of PBAT film and molded product have indicated that significant biodegradation occurs in one year in soil, sea water and water with activated sludge. On contrary, even though PLA has good melt processibility, strength and biodegradation/composting properties, it has both low flexibility and low impact strength. At this point, the flexibility, softness and impact strength of the final product can be improved by mixing PBAT with PLA. Some studies show that the least compatible blending ratio of PBAT and PLA is 50/50. However, it has been shown that miscibility and thus the mechanical property of a 50/50 blend of PBAT and PLA are improved by applying ultrasound energy to the melt blend with an ultrasonic device for 20 to 30 seconds. In this study, tensile strength is found to increase with increasing sonication time. Specifically, tensile strength reaches the highest value up to 20 seconds and then decreases after 20 seconds, whereas impact strength increases up to 30 seconds and then decreases over time after that point. However, sonicated system is found to have much higher impact strength than that of an un-sonicated system. It is explained that excess energy is consumed by the plastic deformation of PBAT phase in sonicated system, while propagating stress passes around the PBAT phases since they are immiscible and separated in the untreated system. This can be seen from a scanning electron microscopy (SEM) that, a minimum domain size of 4.7 μm is achieved after 30 seconds of sonication but notably increased with time afterwards. It is concluded that the excess energy leads to the flocculation of domain (Lee, Sangmook, Youngjoo Lee and Jae Wook Lee, “Effect of Ultrasound on the Properties of Biodegradable Polymer blends of Poly(lactic acid)with Poly(butylene adipate-co-terephthalate,” Macromolecular Research, Vol. 15, No. 1, pp 44-50 [2007]). As pointed out above, PBAT has excellent elongation at break of above 500%. On contrary, the elongation at breaks are only 9% and 15% for PLA and PHBV (“Biodegradable polyesters: PLA, PCL, PHA”. . . , http://www.biodeg.net/bioplastic.html). Therefore, in addition to increasing the flexibility, extensibility and softness of film, packaging material and fabric made by blending PBAT with PLA or PHA, a laminate with good extensibility can be produced by the lamination of PBAT film into elastic biodegradable or non-biodegradable fabrics. The chemical structure of PBAT is shown below:

PBAT is available commercially from BASF as Ecoflex™, Eastman Chemical as Easter Bio®, and from Novamont of Italy as Origo-Bi®. DuPont is marketing a biodegradable aromatic copolyester known as Biomax®. However, rather than PLA, it is a modified polyethylene terephthalate) with a high content of terephtalic acid and a high temperature of about 200° C. Like PLA, Biomax® must firstly undergo hydrolysis before biodegradtion, which begins with small molecules being assimilated and mineralized by some microorganisms existed in the nature (Vroman, Isabelle and Lau Tighzert,“Biodegradable Polymers,” Materials 2009, 2, 307-344). In 2004, Novomont purchased the Eastar Bio copolyester business from Eastman chemical Company (“Novamont buys Eastman's Eastar Bio technology” http://www.highbeam.com/doc/1G1-121729929.html). BASF notes that its PBAT, Ecoflex™ is highly compatible with natural materials such as starch, cellulose, lignin, PLA and PHB (“Bio-Sense or Nonsense, ” Kunstoffe International 8/2008 [Translated from Kunstoffe 8/2008, pp. 32-36).
Poly(butylenes succinate) PBS and its copolymer belong to the poly(alkenedicarboxylate) family. They are synthesized by polycondensation reaction of glycol (such as ethylene glycol and 1,4-butanediol) with aliphatic dicarboxylic acid (like succinic acid or adipic acid). They are marketed in Japan by Showa High Polymer as Bionolle® and in Korea by Ire Chemical as EnPol®. Different alkenedicarboxylates that have been produced are PBS, poly(ethylene succinate) (PES) and a copolymer prepared by the addition of adipic acid poly(butylene succinate-co-adipate) or PBSA. In addition, a copolymer made by the reaction of 1,2-ethylenediol and 1,4 butanediol with succinic and adipic acids has been marked in Korea by SK Chemical as Skygreen®. Another alipahatic copolyester sold by Nippon Shokubai of Japan is known as Lunare SE®. PBS is a crystalline polymer with a melting point of 90-120° C. and a glass transition temperature (Tg) of about −45° C. to −10° C. The PBS has the Tg value between those of polyethylene (PE) and polypropylene (PP), and it has similar chemical properties to those of PE and PP. Besides, the PBS has a tensile strength of 330 kg/cm2 and an elongation-to-break of 330%, while its processibility is better than that of PLA (Vroman, Isabelle and Lau Tighzert,“Biodegradable Polymers,” Materials 2009, 2, 307-344). The chemical structure of PBS is shown below:

PBS consisted of succinic acid may also be produce by bacteria. At this point, bio-based succinic acid is used by Sinoven Biopolymers of China to produce PBS with a renewable content of 50%. It is reported that this kind of PBS has better performances than any other biodegradable polymers and has a heat resistance above 100° C. (“Production of Bio-based polybutylene succinate (PBS)”, http://biopol.free.fr/index.php/production-of-biobased-polybutylene-succinate-pbs/). PBS is blended with PLA to improve flexural properties, heat distortion temperature, impact strength and gas permeability. Herein, PBS can be miscible with PLA and reduce the brittleness of PLA when the concentration of PBS is less than 20% (Bhatia, Amita, Rahul K Gupta, Sati N Bhattacharya and H. J. Choi, “Compatibility of biodegradable poly (lactic acid) (PLA)and poly (butylenes succinate) (PBS) blends for packaging application,” Korea-Australia Rheology Journal, November 2007, Vol. 19, No. 3, pp. 125-131).